Millions of Americans could be walking around with tiny time bombs in their hearts.

The concern centers on devices called drug-eluting stents. Doctors implant them in the hearts of about a million Americans a year to treat coronary artery disease. They generate some $5 billion a year in sales for the two companies that make them. But they may be doing more harm than good.

Next month a panel of experts will try to advise the Food and Drug Administration on what to do about it. But many top doctors and scientists admit they are in uncharted waters with a frightening problem that was largely unanticipated. By one estimate the devices already kill 2,000 Americans a year — and no one knows what the long-term danger will be.

To understand the potential hazard, it helps to look at the history of efforts to open the arteries to the heart when they get clogged with cholesterol-containing plaque. That blockage leads to shortness of breath and the chest pains called angina. If the artery closes completely the result is a heart attack with destruction of heart muscle and often death.